Organic light-emitting diode (OLED) display devices utilize OLEDs to display images. Such display devices are active devices which differ from traditional thin-film-transistor liquid-crystal display (TFT-LCD) devices in actively emitting light and not requiring backlight. They have many advantages such as high contrast, fast response and small thickness, and are praised as display devices of the next generation that will replace the TFT-LCD devices.
Depending on how they are driven, OLED display devices can be categorized into passive matrix organic light-emitting diode (PMOLED) devices and active matrix organic light-emitting diode (AMOLED) devices.
An AMOLED display device comprises scan lines, data lines and an array of pixels defined by the scan lines and data lines. Each of the pixels in the array includes an OLED and a pixel circuit that drives the OLED. Reference is now made to FIG. 1, which is a diagram showing a pixel circuit in an AMOLED display device of the prior art. As shown in FIG. 1, the conventional pixel circuit 10 generally includes a switch thin-film transistor T1, a drive thin-film transistor T2 and a capacitor Cs. The switch transistor T1 is connected to a scan line S(n). When the switch transistor T1 is turned on via the scan line S(n), a data voltage Vdata provided by a data line is stored via the switch transistor T1 in the capacitor Cs, thereby causing the drive transistor T2 to produce a current which drives the OLED to emit light.
The brightness of the pixel is determined by the current flowing through the OLED, and the current is in turn under the control of the pixel circuit. In this conventional pixel circuit, the current flowing through the OLED is affected by a threshold voltage of the drive transistor and a power supply voltage VDD applied to the pixel circuit. Upon a change occurring in the threshold voltage of the drive transistor or in the power supply voltage VDD, the current flowing through the OLED may undergo a significant variation which can lead to the OLED emitting light with a different brightness level from those of other OLEDs in response to their corresponding data signals which, however, indicate the same brightness level. Therefore, it is difficult for this conventional AMOLED display device to display an image with uniform brightness.
Therefore, there is an urgent need in this art for a solution to address the problem of low brightness uniformity of conventional AMOLED display devices.